Field of the Invention
The present invention relates to a polishing apparatus and a polishing method for polishing a substrate, such as a wafer.
Description of the Related Art
From a viewpoint of improving yield in fabrication of semiconductor devices, management of surface conditions of a peripheral portion of a substrate has been attracting attention in recent years. In the fabrication process of the semiconductor devices, various materials are deposited on a silicon wafer to form a multilayer structure. As a result, unwanted films and roughened surface are formed on a peripheral portion of the substrate. It has been a recent trend to transport the substrate by holding only its peripheral portion using arms. Under such circumstances, the unwanted films remaining on the peripheral portion would be peeled off during various processes and could adhere to devices, causing lowered yield. Thus, in order to remove the unwanted films, the peripheral portion of the substrate is polished using a polishing apparatus.
This type of polishing apparatus polishes the peripheral portion of the substrate by bringing a polishing surface of a polishing tape into sliding contact with the peripheral portion of the substrate. In this specification, the peripheral portion is defined as a region including a bevel portion which is the outermost portion of the substrate and a top edge portion and a bottom edge portion located radially inwardly of the bevel portion.
FIG. 1A and FIG. 1B are enlarged cross-sectional views each showing a peripheral portion of a substrate. More specifically, FIG. 1A shows a cross-sectional view of a so-called straight-type substrate, and FIG. 1B shows a cross-sectional view of a so-called round-type substrate. In the substrate W shown in FIG. 1A, the bevel portion is an outermost circumferential surface of the substrate W (indicated by a letter B) that is constituted by an upper slope (an upper bevel portion) P, a lower slope (a lower bevel portion) Q, and a side portion (an apex) R. In the substrate W shown in FIG. 1B, the bevel portion is a portion (indicated by a letter B) having a curved cross section and forming an outermost circumferential surface of the substrate W. A top edge portion is a flat portion E1 located radially inwardly of the bevel portion B. A bottom edge portion is a flat portion E2 located on the opposite side of the top edge portion and located radially inwardly of the bevel portion B. These top edge portion E1 and bottom edge portion E2 may be collectively referred to as edge portions. The edge portions may also include a region where the devices are formed.
The polishing apparatus has a polishing end point detection unit for detecting a polishing end point of the substrate. This polishing end point detection unit is configured to monitor polishing of the substrate based on a polishing index value indicating a film thickness (e.g., polishing time) and determine the polishing end point.
However, the substrate having a multilayer structure includes different kinds of films formed thereon, and these films have different hardness normally. Therefore, if the polishing end point is controlled based on the polishing time, excessive polishing may occur in a soft film and insufficient polishing may occur in a hard film.
In a conventional polishing apparatus, a pressing member of a polishing head is lowered by an air cylinder to press a polishing surface of a polishing tape against the substrate under a predetermined polishing load, as disclosed in Japanese Laid-Open Patent Publication No. 2012-213849. However, since the air cylinder cannot control a lowered position of the pressing member accurately, an error may occur between a target polishing amount and an actual polishing amount of the substrate.